Method of assembling a semiconductor device package

ABSTRACT

A method of assembling a semiconductor device package includes first attaching a semiconductor device to a die-pad area of a leadframe. Electrical connections are then between electrical contact areas on the semiconductor device and electrical connection areas on the leadframe to form a device/leadframe assembly. An adhesion enhancing coating is then deposited on the exposed surface of the device frame/leadframe assembly before encapsulating the coated device leadframe assembly in an electrically insulating material.

BACKGROUND OF THE INVENTION

The invention relates to a method of assembling a semiconductor devicepackage and in particular, a method of assembly which minimises orprevents delamination at interfaces within the assembled package.

Plastic surface mount semiconductor device packages are susceptible tocracking during solder reflow and this phenomenon is commonly referredto as “popcorn cracking”. Popcorn cracking occurs because epoxy moldingcompounds used to encapsulate semiconductor devices are hygroscopic andreadily absorb moisture from the environment. During solder reflow, highhydrothermal stresses are induced due to the combination of rapidvaporization of the absorbed moisture and mismatches in the coefficientsof thermal expansion between dissimilar materials in the package. Oncethe stress level reaches a critical threshold, delamination occurs,usually at the weakest interface, followed by the build-up of vapourpressure in the delaminated cavity forming a characteristic dome-shapedbulge. This leads finally to cracking of the encapsulation material.

Popcorn cracking is a potential reliability problem as the delaminatedareas and/or cracks can induce corrosion failures, alter the thermalperformance of power devices, and affect the stress distribution andconcentration.

The molding compound/die-pad interface is known to be susceptible todelamination and the source of most popcorn failure modes. The situationis aggravated with the emergence of larger and thinner packages.Moreover, moisture can degrade the polymer/metal interfacial durability,and decrease the fracture toughness of the molding compound at reflowtemperatures. Furthermore, the oxidation of copper-based leadframematerials during the assembly process can result in poor adhesionbetween the molding compound and die-pad. The main cause of pooradhesion has been attributed to the weak copper oxide layer on theleadframe surface. Studies have reported that the adhesion strengthbetween the molding compound and the copper leadframe decreases withincreasing oxide thickness.

Various techniques to enhance molding compound/die-pad adhesion havebeen suggested as possible solutions to prevent popcorn cracking.However, most of the solutions which have been proposed do not eliminatethe problem of popcorn cracking completely and are either noteconomically viable and/or feasible to implement in mass production.

For example, organic adhesion promoters such as silane coupling agentsare widely used in die-attach adhesives and molding compounds to improveadhesion at the various interfaces. However, they are temperaturesensitive and susceptible to degradation at elevated temperatures(typically greater than 200° C.), for example, during wire bonding.

Use of an inorganic zinc-chromium (Zn—Cr) leadframe coating(commercially known as “Olin A2” and supplied by Olin Metal ResearchLaboratories) is believed to be effective in eliminating popcorncracking. However, the Zn—Cr coating is deposited on the leadframeelectrolytically via an electroplating process. However, due to thetemperature stability of the Zn—Cr coating layer, it can prevent orinterfere with subsequent solid-state bonding, fusion or solderingprocesses such as wire bonding, etc. Therefore, it is necessary toeither mask the bonding/soldering areas during the coating process or tosubsequently strip the coating from these areas. Therefore, there aresurfaces of the leadframe and semiconductor device in the finishedpackage which do not have the Zn—Cr coating and are still prone topopcorn cracking.

Chip buffer coatings such as polyimides, are commonly used to minimisethermo-mechanical stresses and delamination at the mold compound/chipinterface. However, polyimides have the disadvantage that they have atendency to absorb moisture and therefore, further contribute to popcorncracking.

BRIEF SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, a method ofassembling a semiconductor device package comprises:

-   -   (i) attaching a semiconductor device to a die-pad area of a        leadframe;    -   (ii) forming electrical connections between electrical contact        areas on the semiconductor device and electrical contact areas        on the leadframe to form a device/leadframe assembly;    -   (iii) depositing an adhesion enhancing coating on exposed        surfaces of the device/leadframe assembly; and    -   (iv) encapsulating the coated device/leadframe assembly in an        electrically insulating material.

In accordance with a second aspect of the present invention, asemiconductor device package comprises a leadframe; a semiconductordevice attached to a first portion of the leadframe; electricalconnections extending from electrical contact areas on the semiconductordevice to electrical contact areas on a second portion of the leadframe;an adhesion enhancing coating on surfaces of the leadframe, theelectrical connections and the semiconductor device; and an electricallyinsulating material encapsulating the semiconductor device, theelectrical connections and the first and second portions of theleadframe.

An advantage of the invention is that by depositing an adhesionenhancing coating on the leadframe and semiconductor device after theelectrical connections are formed between the semiconductor device andthe leadframe, the adhesion enhancing coating is also deposited on theelectrical connections between the semiconductor device and theleadframe. There is also the advantage that if the adhesion enhancingcoating is deposited electrolytically via an electroplating process, theelectrical connections provide a conduction path from the leadframe tothe semiconductor device to enable electrolytic deposition of theadhesion enhancing coating on electrically conducting surfaces of thesemiconductor device.

Preferably, the adhesion enhancing coating is a metallic coating and istypically deposited electrolytically via an electroplating process.

Preferably, the metallic coating is an inorganic Zn—Cr coating, such asOlin A2.

Typically, the semiconductor device may be attached to the leadframe byan epoxy die-attach adhesive or with solder die-attach.

Preferably, the semiconductor device package is a surface mountsemiconductor device package.

An example of a method of assembling a semiconductor device package inaccordance with the invention will now be described with reference tothe accompanying drawings, in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view through a surface mount semiconductordevice package having an adhesion enhancing coating in accordance withthe prior art;

FIG. 2 is a cross-sectional view through a surface mount semiconductordevice package in which a semiconductor device is attached to aleadframe with epoxy die-attach adhesive and incorporating an adhesionenhancing coating in accordance with the invention;

FIG. 3 is a cross-sectional view through a surface mount semiconductorpackage in which a semiconductor device is attached to a leadframe withsolder and incorporating an adhesion enhancing coating in accordancewith the invention; and

FIG. 4 is a cross-sectional view through a surface mount semiconductorpackage including a power semiconductor device with a protruding heatsink.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view through a surface mount semiconductordevice package 1 in which a leadframe 2, which comprises a die-pad area2 a and bonding leads 2 b, has been coated with a Zn—Cr coating 3 (suchas Olin A2) which is deposited on the leadframe electrolytically via anelectroplating process prior to attachment of a semiconductor device 4to the die-pad area 2 a.

During deposition of the coating 3 it is necessary to either maskbonding areas 7 of the bonding leads 2 b to which the wires 5 are to bebonded, or to carry out the deposition of the Zn—Cr coating 3 by a twostep plate and strip process. Generally it is also necessary to mask orstrip a portion of the die-pad area 2 a to permit establishment of anelectrical ground contact between the semiconductor device 4 and thedie-pad area 2 a.

After the masking has been removed from the leadframe 2, or the coating3 has been stripped from the relevant areas, the semiconductor device 4is attached to the die-pad portion 2 a by an epoxy die-attach adhesive6. The leadframe 2 with the semiconductor device 4 attached is thenpassed to a wire bonding process in which wires 5 are bonded between thebonding areas 7 on the bonding leads 2 b and electrical contact areas onthe semiconductor device 4.

Generally, the leadframe 2 is only one of a number of leadframes 2 whichare joined together to form of a strip or matrix of leadframes 2. Eachleadframe 2 on the strip or matrix comprises a die-pad area 2 a andrespective bonding leads 2 b. Each die-pad area 2 a on the leadframe 2will have a semiconductor device 4 attached to it by a die-attachadhesive 6. Having the leadframes 2 in a strip or matrix which carries anumber of semiconductor devices 4 permits easier handling of theleadframes 2 during assembly of the semiconductor device package 1.

After the wire bond process, the strip or matrix of leadframes 2 ispassed to a molding process where an electrically insulatingencapsulation material 8 is molded around the die-pad area 2 a, bondingareas 7, semiconductor device 4 and wires 5 to leave only extremities ofthe bonding leads 2 b protruding from the encapsulation material 8.

After the molding process, the leadframes 2 are singulated to separateeach semiconductor device 4 and the respective die-pad area 2 a andbonding leads 2 b from adjacent semiconductor devices 4 and respectivedie-pad areas 2 a and bonding leads 2 b. Optionally, after the moldingprocess, the coating can be removed from the extremities of the bondingleads 2 b and/or from a protruding heat sink (if present).

There are a number of disadvantages associated with this prior artcoating process. In particular, the requirement to either mask portionsof the leadframe 2 or to subsequently strip coating from areas such asthe bonding areas 7 and the ground bond area on the die-pad area 2 a. Inaddition, this prior art coating process cannot coat the surface of thesemiconductor device 4. Hence, this conventional assembly method doesnot prevent delamination and subsequent popcorn cracking at interface 9between the molding compound 8 and the semiconductor device 4.

FIG. 2 shows a cross-sectional view of a semiconductor device package 10which includes the same components as the semiconductor device package 1and the same items are identified using the same reference numerals asin FIG. 1. However, the process for assembly of the semiconductor devicepackage 10 is slightly different from that used for the semiconductordevice package 1 shown in FIG. 1.

In the assembly of the package 10, the semiconductor device 4 isattached to the die-pad portion 2 a by an epoxy die-attach adhesive 6before the Zn—Cr coating 3 is deposited. As in the example describedabove, the leadframe 2 is initially one of a number of leadframes 2joined together to form a strip or matrix. After the semiconductordevice 4 has been attached to the die-pad portion 2 a, the strip ormatrix of leadframes 2 with the attached semiconductor devices 4 ispassed to a wire bonding process for bonding of the wires 5 betweencontact areas on the semiconductor device 4 and the bonding areas 7 onthe bonding leads 2 b.

After the wire bonding process, the strip or matrix of leadframes 2,with the attached semiconductor device 4 and the wire bonds 5, isimmersed in an electroplating bath and the Zn—Cr coating 3 is depositedelectrolytically on the leadframe 2 in the areas shown, on the wirebonds 5 and electrically conducting surfaces on the semiconductor device4 by an electroplating process.

After deposition of the Zn—Cr coating 3, the electrically insulatingencapsulation material 8 is molded around the semiconductor devices 4,leadframes 2 and wire bonds 5, as in the prior art package 1. The stripor matrix of leadframes 2 is then singulated into individualsemiconductor device packages.

FIG. 3 shows a semiconductor device package 30 which has been assembledusing a process similar to that used for assembly of the semiconductordevice package 10. However, the semiconductor device 4 is attached tothe die-pad area 2 a by solder 31. Therefore, as the solder 31 iselectrically conducting, the Zn—Cr coating 3 is also deposited along thesides of the solder 31.

FIG. 4 shows a semiconductor device package 40 which has been assembledusing a process similar to that used for assembly of the semiconductordevice packages 10, 30. However, the package 40 includes a powersemiconductor device 41 attached to a thickened die-pad area 2 a bysolder 31. Therefore, as the solder 31 is electrically conducting, theZn—Cr coating 3 is deposited along the sides of the solder 31, as in thepackage 30. In addition, as a bottom surface 43 of the die-pad area 2 cis not covered by the material 8, and the solder 31 is thermallyconductive, the die pad area 2 c acts as a heat sink for the device 41.

In the device packages 10, 30, 40, the coating is removed fromnon-encapsulated portions of the leadframe 2. In the packages 10, 30 thecoating is removed from the non-encapsulated portions of the bondingleads 2 b and in the package 40 the coating is removed from thenon-encapsulated portions of the bonding leads 2 b and the surface 43 ofthe die-pad area 2 c. However, removal of the coating from these areasis not essential and is only a possible option.

By depositing the Zn—Cr coating 3 after attachment of the semiconductordevices 4, 41 to the die-pad areas 2 a, 2 c of the leadframe 2 andformation of the wire bonds 5, it is not necessary to mask any areas ofthe leadframe 2 during the coating process.

In addition, by depositing the coating 3 after wire bonding, but beforemolding, there is the advantage that the entire wire bonded assembly issubmerged into the plating bath for coating deposition, and as the wires5 establish electrical paths from the leadframe 2 to the electricalcontact surfaces of the semiconductor device 4, the Zn—Cr coating 3 isalso deposited on the electrical contact surfaces of the semiconductordevice 4. The degree of coating coverage on the surface of the device 4is dependent on the availability of electrically conducting paths on thesurface. This is influenced by device design, distribution ofmetallisation, type of device passivation, provision ofintrinsic/extrinsic passivation and provision of intrinsic/extrinsicelectrical contact.

As an alternative to electrolytical deposition, it is possible that thecoating could be deposited by a wet chemical catalytic process or a dry,physical/chemical deposition. In addition, although the deposition of aZn—Cr coating has been described any metallic or non-metallic coatingcould be deposited using the invention.

1. A method of assembling a semiconductor device package comprises: (i)attaching a semiconductor device to a die-pad area of a leadframe; (ii)forming electrical connections between electrical contact areas on thesemiconductor device and electrical contact areas on the leadframe toform a device/leadframe assembly; (iii) electrolytically depositing anadhesion enhancing coating by an electroplating process on the entireexposed surfaces of the device/leadframe assembly, the enhancing coatingbeing in the form of an inorganic material comprising metal oxide; and(iv) encapsulating the coated device/leadframe assembly in anelectrically insulating material.
 2. A method according to claim 1,wherein the semiconductor device package is a surface mountsemiconductor device package.
 3. A method according to claim 1, furthercomprising, after encapsulating the coated device/leadframe assembly,removing the coating from non-encapsulated portions of the leadframe. 4.A semiconductor device package comprises a leadframe; a semiconductordevice attached to a first portion of the leadframe; electricalconnections extending from electrical contact areas on the semiconductordevice to electrical contact areas on a second portion of the leadframe;an adhesion enhancing coating on the entire surfaces of the leadframe,the electrical connections and the semiconductor device; and anelectrically insulating material encapsulating the semiconductor device,the electrical connections and the first and second portions of theleadframe, wherein the enhancing coating is in the form of an inorganicmaterial comprising metal oxide.
 5. A semiconductor device packageaccording to claim 4, wherein the enhancing coating is an inorganicZn—CR coating.
 6. A semiconductor device package according to claim 4,wherein the semiconductor device is attached to the first portion of theleadframe by an adhesive.
 7. A semiconductor device package according toclaim 4, wherein the semiconductor device is attached to the firstportion of the leadframe by solder.
 8. A semiconductor device packageaccording to claim 4, wherein the semiconductor device is a surfacemount semiconductor device.
 9. A semiconductor device package accordingto claim 4, wherein the semiconductor device is a power semiconductordevice.
 10. A semiconductor device package according to claim 4, whereinthe first portion of the leadframe forms a heat sink for thesemiconductor device and a surface of the first portion is not coveredby the electrically insulating material.